The present invention relates to a method of radio communication in a system comprising a plurality of communicating modules, and an associated radio communication system.
The invention lies in the field of wireless communication performed by communicating modules, especially in electrical installations, the communicating modules being for example modules for measuring one or more physical quantities of the electrical installation.
There exist short-range radio communication protocols, for example Bluetooth (trademark) and ZigBee, adapted for domestic networks.
Diverse communicating modules, for example electrical measurement sensors, temperature or pressure sensors, implement the ZigBee protocol based on the IEEE 802.15.4 communication protocol. The ZigBee protocol exhibits the advantage of reducing the electrical consumption to the strict minimum, while allowing a low data transmission rate which is nevertheless sufficient for the transmission of equipment measurement and control data.
The objective of the ZigBee Green Power protocol is to allow communications between modules with a yet further reduced consumption of electricity.
For example, a system for monitoring and managing electrical installations is considered, comprising a plurality of communicating modules, for example according to the ZigBee Green Power protocol, each adapted to communicate in a bidirectional manner with a hub device, which receives the totality of the measurement information transmitted by the diverse modules and is adapted to aggregate them for subsequent use. From the communications point of view, each communicating module is a node of a star network, centred around the hub device.
In such a communications network, one of the problems which arises is to ensure that each of the communication nodes can send radio frames or messages, without interference or collision with one or more other radio frames sent by other nodes, so as to ensure sufficient quality of service.
A communication scheme known by the name TDMA (for “Time Division Multiple Access”) consists in allocating each node a send time interval, repeated periodically.
Each communicating module forming a node of the communication network possesses an internal clock, quartz-based or formed by an RC oscillator, and uses this internal clock to regulate message sends.
However, there is a risk of temporal drift inducing possible temporal superpositions of the send intervals associated with distinct communication nodes. Such a superposition induces a risk of collision, and consequently of loss of one or more messages.
Moreover, such a scheme requires specific intervention for sequencing the time intervals when bringing a plurality of communicating modules into service.
A conventional protocol, the Aloha protocol, making it possible to ensure good quality of service, consists in performing the transmission by the receiver node of an acknowledgement message (or ACK), and, in case of non-receipt of an acknowledgement message, the sender node waits a random time before re-sending the initial message.
On the one hand, when the sender nodes are communicating measurement modules, such as sensors, they do not possess enough energy to remain in permanent or prolonged reception, therefore the listening periods in reception are reduced.
Moreover, more generally, because of the selection of a random waiting time, the risk of collision is increased, therefore the communications system is unstable. Moreover, the time required for each node to find an appropriate send time interval may be very long, and does not have any guaranteed upper limit. Stated otherwise, the system convergence time may turn out to be long.